Currently nonhuman primates are the only species that are susceptible to simian immunodeficiency virus (SIV) and develop a disease nearly identical to that seen in humans infected with HIV. In humans infected with HIV the individual response to infection is variable and particular patients demonstrate marked variability in chronologic progression to clinical AIDS. The time interval between infection and clinical disease is highly variable and depends on both viral and host factors. A small percentage of infected patients apparently are able to resist disease progression; research into the viral/host factors may provide information of clinical and therapeutic importance and these long-term survivors may be an important group to study. We currently have 4 M. nemestrina that have been infected with SIV/HIV for years and fall into the category of long-term survivors. None of these animals has yet progressed to clinical AIDS. These animals are important to maintain for f uture rese arch and provide an important animal model for disease progression and pathogenesis in individuals surviving infection for long periods of time. One M. nemestrina was inoculated with SIVMne E11S in 1988 and has remained seronegative although virus was isolated from this animal at 174 and 195 weeks post-inoculation. This animal has developed a significant decline in CD4+ cells but has yet to develop disease. In addition, 3 M. nemestrina were inoculated with HIV-1 as reported in 1992 and virus has not been isolated since 10 weeks post-inoculation. Two of the animals remain strongly seropositive, which provides strong evidence for continued viral replication. To date these animals have not progressed to clinical disease. FUNDING NIH grant RR00166.